Production facilities for making asphalt concrete to be used as a paving composition are well-known. Feed materials for these facilities include aggregate materials and asphalt cement. The aggregate materials may be provided in the form of virgin aggregate materials, and/or RAP, and/or RAS. If RAP and/or RAS are included in the feed materials, these components will also provide an additional source of asphalt cement.
Some conventional asphalt concrete production plants employ a rotating dryer drum in which virgin aggregate materials and/or RAP and/or RAS are introduced. A burner is located at one end of the drum and the input feed materials are moved along the drum through the heated gases generated by the burner in either parallel flow or counter-current flow to an outlet. A separate mixer, such as a rotating drum mixer or a pugmill, is employed to mix the heated and dried aggregate materials with liquid asphalt cement. Another type of asphalt concrete production plant employs a dryer/mixer that dries and heats the aggregate material and also mixes it with asphalt cement. One such type of dryer/mixer is the DOUBLE BARREL® brand dryer/mixer that is sold by Astec, Inc. of Chattanooga, Tenn. This dryer/mixer includes a generally cylindrical fixed outer drum and a heating chamber comprised of a generally cylindrical inner drum that is adapted to rotate with respect to the outer drum. A burner at one end of the inner drum heats aggregate material by direct exposure to the hot gases generated, and the heated aggregate material is discharged from the inner drum into the outer drum where it is mixed with asphalt cement and/or with RAP and/or RAS. If substantial quantities of RAP and/or RAS are introduced into the DOUBLE BARREL® brand dryer/mixer, a separate mixer such as a pugmill or mixing drum may be employed to add and incorporate additional asphalt cement into the mixture.
Because some conventional systems expose liquid asphalt cement and/or RAP and/or RAS aggregate materials to the high-temperature gases used for drying and heating the aggregate materials and to the steam generated in the drying process, emissions of smoke and volatile organic components (“VOC”) are stripped from the light oil fractions of the asphalt cement components. In order to prevent these emissions from being discharged to the atmosphere, it has been deemed desirable, when only virgin aggregate materials are used, to either direct the emissions into the burner for incineration, or to filter the emissions from the plant exhaust gases and condense them for disposal. Even though counter-current flow is more thermally efficient than parallel flow, conventional asphalt concrete production plants that process aggregate materials containing a high percentage of RAP and/or RAS are generally operated in a parallel heat flow arrangement, where the aggregate materials to be heated and dried are carried through the dryer in the same direction as the heating gases, in order to minimize smoke and VOC emissions. In addition, exposure of high proportions of RAP and/or RAS aggregate materials to the high-temperature gases used for drying and heating the aggregate materials and to the steam generated in the drying process causes oxidation of the liquid asphalt on the RAP and/or RAS, which results in degrading the asphalt and any pavement materials made with it. This reduces the number of applications for which high-RAP content or high-RAS content asphalt concrete is considered suitable. Finally, conventional equipment that is used to make high-RAP content or high-RAS content asphalt concrete must generally be operated at a lower production rate than when the same equipment is used to make asphalt concrete with only small amounts of RAP and/or RAS or with all virgin aggregate materials.
It would be desirable if a method and apparatus for producing asphalt concrete from aggregate materials including a high percentage of RAP and/or RAS could be provided that would limit the emission of undesirable smoke and VOC. It would also be desirable if such a method and apparatus could be provided that would be more thermally efficient than conventional systems. It would also be desirable if such a method and apparatus could be provided that would minimize the oxidation of asphalt cement in high-RAP content and/or high-RAS content asphalt concrete, thereby making such products suitable for more paving applications. Furthermore, it would be desirable if such a method and apparatus could be provided that would allow the production of high-RAP content and/or high-RAS content asphalt concrete at production rates that are comparable to those obtained when only virgin aggregate materials are used.